A tire pressure monitoring system can be used to inform a vehicle driver of a tire condition problem such as improper tire pressure and/or temperature. Improper inflation pressure information is particularly useful to the driver of a vehicle having “run flat” tires. Before the use of run flat tires, in the event of sudden tire pressure lose (e.g. a tire is punctured) while driving, the driver could usually feel the condition due to a change in the handling characteristics of the vehicle. However, with the advent of the run flat tires, a driver might not detect sudden tire pressure lose. However, it is still important that the driver be informed, even with a run flat tire, when tire pressure and/or temperature values are not within predetermined limits.
To implement a tire pressure monitoring (“TPM”) system, a vehicle based electronic control unit (“ECU”) can control a plurality of low frequency (“LF”) transmitters, each wheel well having an associated LF transmitter. The LF transmitters transmits a LF signal to an associated TPM sensor mounted within its associated tire assembly secured to the wheel, for example. When the LF signal is received by each TPM sensor, the TPM sensor will process the LF signal, sense the pressure of its, tire, and transmit a radio frequency (“RF”) signal back to the vehicle based ECU. The ECU will process the RF signals from all the TPM's of the tires and provide an output signal to a display that is viewable by the vehicle's driver that will indicate an abnormal pressure status of any of the tires on the vehicle.
In some instances, the LF signal provided to each TPM sensor is in the form of an amplitude shift-keying (“ASK”) modulated signal. The TPM sensor would include a demodulator that can demodulate the ASK signal such that the rising edges of the ASK signal can be detected. However, in other instances, the LF signal provided to each TPM sensor is an unmodulated continuous wave (“CW”) signal. Since the CW signal is unmodulated, a TPM sensor would not be able to detect a rising edge of the CW signal if the CW signal is present before the TPM sensor has already been activated, as a demodulated CW wave is a substantially constant waveform. If the CW signal is not present when the TPM sensor is initially activated, then the TPM sensor will detect exactly one rising edge, at the time when the TPM sensor first receives the CW signal. However, in some applications, the CW signal is present before the TPM sensor is activated, such that the TPM sensor will never detect the CW wave. In other applications, the TPM sensor may be activated before the CW signal is first received, but as stated above, the TPM sensor will detect only one rising edge, while many applications would require multiple rising edges to be detected.
Previous attempts to design a TPM sensor that can detect an LF CW signal have proven to be expensive. Such previous attempts require a separate circuit specifically designed to detect the CW signal, and such circuits usually require a significant number of components and occupy valuable space on a printed circuit board (“PCB”). What is needed is a TPM sensor that can be programmed to detect both, amplitude modulated (“AM”) signals, and CW signals.